1. Technical Field
The present invention relates to a zoom lens which is to be suitably used on a small-sized appliance having an imaging function, particularly on a digital still camera, a portable cellular phone with camera, a personal digital assistant (PDA) or the like.
2. Related Art
Recently, the imaging device, such as the digital still camera, is demanded to be reduced wholly in size as size reduction proceeds for the imaging device, such as the CCD (charge coupled devices) and the CMOS (complementary metal oxide semiconductor). In such a situation, development has been made on a structure reduced in thickness in order to be incorporated in the imager device by bending the optical path intermediately of its lens system into so-called a bending optical system.
As a zoom lens using a bending optical system, Patent Document 1 (JP-A-2000-131610) describes a zoom lens structured with a first lens group having a positive refractive power, a second lens group having a negative refractive power, a third lens group having a positive refractive power and a fourth lens group having a positive refractive power in order from an object side and magnification of the zoom lens is to be varied by moving the negative second lens group and the positive fourth lens group. In this zoom lens, the optical path is bent at approximately 90 degrees by arranging a prism in the first lens group. In the first lens group, a fixed positive lens group is arranged in back of the prism. Meanwhile, Patent Document 2 (JP A 2004-205796 corresponding to U.S. Pat. No. 7,079,328 B2) describes a zoom lens structured with a first lens group having a negative refractive power, a second lens group having a positive refractive power, a third lens group having a negative refractive power and a fourth lens group having a positive refractive power, in the order from the object side and magnification of the zoom lens is to be varied by moving the positive second lens group and the negative third lens group. In this zoom lens, the optical path is bent at approximately 90 degrees by arranging a prism in the first lens group in a position closest to the object. In the first lens group, a fixed lens group is arranged in back of the prism.
As described in Patent Documents 1 and 2, the conventional bending type zoom lens is arranged with a fixed lens group in back of the optical-path bending prism in the first lens group, and the zooming group in back thereof. Particularly, it is often the case to arrange a positive lens group fixed upon zooming, in back of an optical-path bending prism in the first lens group and a zooming group in back thereof. The zooming group, in many cases, is structured with a negative lens group and a positive lens group in order from the object side. In the meanwhile, where a bend-type zoom lens is used in the personal digital assistant application, the entire volume is required small in addition to the reduced thickness as a lens module. However, the conventional bending type zoom lens is designed intended mainly for its radial (bent direction orthogonal to the optical axis) size in the viewpoint of thickness reduction. Less or no consideration is paid for the length in a direction of the bent optical axis. This is because of the recognition that, in the bend-type zoom lens, shortening is fully done by bending the optical path. However, it is becoming important to reduce the overall length in order to reduce the entire volume. Therefore, for the bending type zoom lens, development is desired for a lens reduced in overall length.
The present invention, made in view of the foregoing problem, aims at providing a zoom lens that the overall length can be reduced while maintaining a well optical characteristic as compared to the conventional bending type zoom lens arranged with a fixed lens group between an optical-path bending prism and a zooming group.